The present invention relates to a method for adjusting the data transmission rate in a fieldbus system that is suitable for controlling safety-critical processes and that comprises at least one subscriber connected to a fieldbus. The invention further relates to a fieldbus system for controlling safety-critical processes having a fieldbus to which at least one subscriber is connected.
Fieldbus systems of the afore-mentioned kind have been used for a long time in most different fields and for most different purposes. For example, a safety device having a microprocessor is disclosed in the published application DE 42 42 936 A1 in which a fieldbus is proposed for transmission of data. Further systems are disclosed in EP 0 559 214 A1, U.S. Pat. Nos. 4,835,850, 5,446,846 or 4,825,362.
The term fieldbus system is generally understood as a system for data communication to which, ideally, any desired number of subscribers can be connected that communicate with each other via the common fieldbus. The communication between the subscribers via the fieldbus is governed by specified protocols. Such a communication system is in contrast to a individual point-to-point communication link between two subscribers where other subscribers are completely excluded from the communication. Examples of known fieldbus systems are the so-called CANbus, the so-called Profibus or the so-called Interbus.
Although the use of fieldbuses offers numerous advantages, mainly with respect to the high wiring effort that would otherwise be required. It was not possible herebefore to employ fieldbuses in practical use for controlling safety-critical processes. The reason is that due to the structure being freely accessible for any subscriber, the degree of fail safety necessary for controlling safety-critical processes could not be guaranteed. The applicant has however developed a fieldbus system in the meantime which also meets the demands for safety-critical processes.
The term safety-critical process is understood in the present invention to describe a process which, in case of a fault, would present a risk for people and goods that may not be neglected. Ideally, it must be hundred percent guaranteed for any safety-critical process that the process will be transferred to a safe state in case a fault should occur. Such safety-critical processes may also be partial processes of larger, higher-level overall processes. Examples for safety-critical processes are chemical processes, where it is an absolute necessity to keep critical parameters within predetermined limits, or complex machine controls, such as the control of a hydraulic press or of an entire production line. In the case of a hydraulic press, for example, the material feeding process may be a non-safety-critical partial process, whereas the process of starting the pressing tool may be a safety-critical partial process as part of the overall process. Other examples of (partial) safety-critical processes are the monitoring of guards, protective doors or light barriers, the control of two-hand switches or the reaction to emergency stop devices.
One of the most important demands to a fieldbus system for controlling safety-critical processes is a defined and fast response time which does not play a role in known systems for data transmission, for example via modem. Such a fieldbus system must be able to stop or terminate the process within a predetermined defined response time, for example in response to pushing an emergency shut-down switch as to avoid any possible damage. The achievable response time mainly depends on the transmission rate of the fieldbus system. A high data transmission rate results in a short response time, since the load of the fieldbus decreases compared with the lower data transmission rate with the same number of subscribers. Hence, also the time period is reduced for which a subscriber has to wait at the most before the fieldbus is enabled for its own transmission of data.
Due to that, it is desirable to operate with a very high data transmission rate. However, this leads to the problem that the transmission quality between the transmitter and the subscriber being located farther away decreases when the data transmission rate is increased.
It is therefore apparent that the adjustment of the data transmission rate in a fieldbus system for controlling safety-critical processes plays a very important role. Approaches for adjusting the data transmission rate in a bus system are for example disclosed in EP 0 896 449 A2, U.S. Pat. Nos. 5,124,943 or 5,881,240.
Typically, the data transmission rate in a fieldbus system is adjusted manually by providing respective adjustment devices at the subscribers of the fieldbus system. These adjustment devices are for example provided as DIP-switches.
It is obvious that the adjustment of the data transmission rate by doing so is very complex and susceptible to faults. Particularly in large fieldbus systems having a plurality of subscribers, it is not unlikely that the data transmission rate will be adjusted at one subscriber incorrectly by mistake. This leads to the result that the subscriber is not able to communicate via the fieldbus anymore. This could lead to fatal results when used for safety-critical processes.
Since the maximum possible data transmission rate decreases with the length of the data transmission path, it is often necessary to reduce the data transmission rate when extending a fieldbus system already present so that also the most distant subscriber is still able to communicate. However, this has the effect that the data transmission rate has to be reduced at all subscribers of the fieldbus system, since the subscribers connected to the fieldbus must transmit generally with same data transmission rate.